Various electrophotographic techniques have been proposed as disclosed, e.g., in U.S. Pat. No. 2,297,691, JP-B-42-23910, and JP-B-43-24748 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Electrophotography generally comprises forming an electrostatic latent image on a photoreceptor by utilizing a photoconductive substance, developing the latent image with toner, and after the toner image is transferred to paper, etc., fixing the toner image by applying heat, pressure or solvent vapors.
Color copying by electrophotography is carried out by repeatedly exposing an original to light through a color separation filter and developing each electrostatic latent image with a corresponding color toner, e.g., yellow, magenta, and cyan color toners, to form a color image comprising a plurality of toners images overlapped.
Color toner comprise a binder resin having dispersed therein colorants, e.g., dyes and pigments, which are finely ground to powder of about 1 to 30 .mu.m. Such toner powder is used as a mixture with a carrier substance such as glass beads, iron powder, or fur.
The above-described color toners as well as a generally employed black toner are required to have (1) excellent tribo chargeability, (2) no dependence on environmental conditions such as temperatures and humidities (i.e., characteristics unchangeable with changes of the environment), and (3) durability (i.e., resistance to deterioration on repeated use). In addition, the color toners are particularly required to have (4) satisfactory spectral reflection characteristics for obtaining a reproduced image with high fidelity, (5) high transparency for allowing overlapping of a plurality of colors, and (6) satisfactory color mixing properties with other color toners.
Among these requirements, transparency of toners after image fixing has a great influence on not only color mixing properties in overlapping multicolor toners but also color developability of an OHP-projected (OHP: overhead projector) image in the case where an image is formed on a transparent resin sheet. Transparency of toners greatly depends on the colorant used. Most pigments used as a colorant fail to exhibit sufficient transparency even though satisfactory in other requirements.
In particular, many acetoacetanilide monoazo pigments, a typical type of yellow pigment, fail to meet the transparency requirement. It has thus been considered impossible to obtain a yellow toner excellent in transparency by using this type of pigment according to the conventional methods for producing toners.
Therefore, other types of pigments have been used as colorants of yellow toners having transparency. However, many of the other pigments are more expensive than acetoacetanilide monoazo pigments, causing a rise of cost of toner production. Moreover, since the choice of the other pigments is limited, it has been difficult to obtain an arbitrary color tone.